Illumination device for backlighting an image reproduction device

ABSTRACT

The invention relates to an illumination device for backlighting an image reproduction device that contains light valves. Said device is characterised in that light spots formed respectively by at least one light emitting diode are arranged in a grid on a thermally conductive support. The respective surface area of the light spots is less than that of the surface area defined by the grid.

The invention relates to an illumination device for backlighting animage reproduction device containing light valves.

Image reproduction devices having light valves, in particular liquidcrystal displays, require sufficiently bright and uniform backlighting.This is achieved by means of fluorescent tubes in the case of relativelylarge displays, such as computer screens for example. The knownillumination devices do not suffice, however, in applications requiringa very high luminance. Thus, a very high luminance is required forso-called head-up displays in motor vehicles, by way of example, sincethe reflected image of the display must still be visible even when thereis high ambient brightness.

Therefore, it is an object of the invention to specify an illuminationdevice which has a high luminance on a given area. This object isachieved according to the invention by virtue of the fact that luminousspots formed by at least one light emitting diode in each case arearranged in grid form on a thermally conductive carrier. In this case,it is preferably provided that the area of the luminous spots is in eachcase less than the area provided by the grid.

The invention advantageously exploits the fact that a multiplicity ofsmall light emitting diodes emit more light than a correspondingly largediode since the entire free surface area of the light emitting diodeemits radiation.

One advantageous development of the illumination device according to theinvention consists in the fact that lines for supplying power to thelight emitting diodes are arranged on an insulating carrier on that areaof the carrier which is not occupied by luminous spots. This enables anadvantageous thermally conductive connection between the light emittingdiodes and the carrier without the lines applied in an insulated mannerimpeding the heat conduction.

This development may advantageously be refined by the lines being routedin a flexible film that is continued as a flat lead outside the carrier.This means that no further contact-connection is necessary apart fromthe contact-connection of the lines to the light emitting diodes withinthe illumination device, which contributes to operational reliabilityand to inexpensive producibility.

In accordance with another development, a particularly good thermallyconductive connection between the light emitting diodes and the carrierconsists in the fact that the luminous spots are applied on submountswhich have good thermal conductivity and are connected to the carrier ina manner exhibiting good thermal conductivity. In this case, it ispreferably provided that the at least one light emitting diode isarranged in an electrically insulated manner on the submount and/or thatthe submounts are composed of silicon.

In order to further improve the heat dissipation, the invention mayprovide for the carrier to be composed of ultrapure aluminum or copperand/or for the carrier to be connected to a heat sink. An example of asuitable heat sink is a large cooling element that emits heat to thesurrounding air over a largest possible surface area. Furthermore,so-called heat pipes are suitable as a heat sink.

Furthermore, the illumination device according to the inventionpreferably provides for interspaces between the luminous spots to befilled with plastic.

In order furthermore to increase the quantity of light radiated, it maybe provided, in accordance with one advantageous embodiment, that aluminous spot is in each case formed by a plurality of light emittingdiodes. For backlighting a monochrome display, the light emitting diodesmay be identically colored in this case. In order to obtain a color thatis not available as a light emitting diode or for backlighting a colordisplay, one development provides for the light emitting diodes of arespective luminous spot to emit varicolored light.

The use of a plurality of light emitting diodes for a respectiveluminous spot has the advantage of a higher luminous efficiency comparedwith a larger light emitting diode. It has proved to be expedient forfour light emitting diodes to form a luminous spot. A different numberof light emitting diodes per luminous spot is also possible, however, inthe context of the invention.

Another advantageous refinement consists in the fact that twogreen-luminous light emitting diodes, one blue-luminous and onered-luminous light emitting diode are provided per luminous spot.Although this emphasizes the green component of the generated lightcompared with the other components, this is necessary in order to obtainwhite, for example approximately 60% green, 25% red and 15% blue.

Arranging the luminous spots on submounts has the advantage that theside walls of the light emitting diodes are elevated completely abovethe lines routed between the luminous spots, so that the radiationemerging therefrom can be utilized. In order to utilize the latter forbacklighting the display, another development of the invention providesfor the luminous spots to be surrounded by a respective reflector. It ispreferably provided in this case that a depression that is formed by thereflector and contains the luminous spot is filled with a transparentplastic.

The light source according to the invention has a high efficiency, longservice life, high reliability in respect of failure, defined emissionand—when varicolored light emitting diodes are used—a narrowbandemission in different colors. It is thus possible to match the spectralemission of the light source and spectral transmission of the colorfilters of the liquid crystal display and to keep down the light lossesdue to the color filters. The high efficiency of the light emittingdiodes results in a high intensity in conjunction with little generationof heat.

The uniform distribution of the color spots over the entire visiblearea, in conjunction with a suitable focusing device, results in afurther effective increase in the luminance. In this case, the uniformdistribution of the luminous spots over the entire visible area resultsin a uniform luminance distribution which can be increased further bythe focusing device. The compact arrangement of the light emittingdiodes in a respective luminous spot results in good color mixing. Thewhite-reflective area and the reflector form mean that light which isthen emitted at a less favorable angle is utilized. The use of verysmall light emitting diodes results in a compact arrangement having athickness of a few millimeters, for example 2 mm.

The light emitting diodes require a very low operating voltage, with theresult that a plurality of light emitting diodes are expedientlyconnected in series. In the event of one of said light emitting diodesfailing, the others are no longer supplied with power and likewise fail.In order to reduce or even to preclude, if appropriate, a visibility ofthis effect, another development of the illumination device according tothe invention provides for the light emitting diodes to be connected toan electric circuit in groups in each case in series.

In order in this case to prevent particularly disturbing stripes or dotsfrom arising in the event of a light emitting diode failing, thisdevelopment may be designed in such a way that the luminous spots whoselight emitting diodes are associated with a respective group areinterleaved with luminous spots of other groups. The interleaving makesit possible, with suitable optical light distribution means, to make thefailure of a group hardly visible.

Another advantageous refinement of this development consists in the factthat when a plurality of identically colored light emitting diodes arepresent per luminous spot, the identically colored light emitting diodesare connected to different electric circuits. Without further measures,a brightness and color change occurs in this case which can be toleratedfor many applications.

However, the color change can be compensated for by virtue of the factthat control devices are provided for the currents fed to the individualelectric circuits, which control devices, in the event of interruptionof one of the electric circuits for identically colored light emittingdiodes, control the currents in the electric circuits for the at leastone other electric circuit for identically colored light emitting diodesand for differently colored light emitting diodes of the same luminousspots in the sense of compensating for the color shift brought about bythe interruption.

Insofar as it is possible with regard to the permissible power loss ofthe affected light emitting diodes, it may be provided in this case thatthe current in the at least one other electric circuit for identicallycolored light emitting diodes is increased. With this measure, both thebrightness and the color can be brought to the original state.

If it is not possible or expedient to increase the power of theremaining identically colored light emitting diodes, then the currentsin the electric circuits for differently colored light emitting diodesmay be decreased. As a result, although the brightness is reduced, thecolor can essentially be maintained. Another advantageous refinement ofthe development consists in the fact that in the case of a grid of 4×8luminous spots having in each case two green-luminous and twored-luminous light emitting diodes, four electric circuits are providedfor the red-luminous light emitting diodes, two electric circuits ineach case being assigned to the light emitting diodes of identicalluminous spots which are distributed over the grid in checkered fashion.In this case, it is preferably provided that the green-luminous lightemitting diodes are connected to eight electric circuits, in each caseone green-luminous light emitting diode of eight luminous spots beingconnected to one electric circuit and a further green-luminous lightemitting diode of the same luminous spots being connected to anotherelectric circuit.

This refinement takes account of the fact that owing to the differentvoltages required for the red-luminous and the green-luminous lightemitting diodes and owing to an expedient operating voltage having amagnitude of approximately 40 V, 16 red-luminous but only eightgreen-luminous light emitting diodes are connected in series.

The invention permits numerous embodiments. One of these is illustratedschematically in the drawing using a number of figures and is describedbelow. In the figures:

FIG. 1 shows a highly diagrammatic illustration of a display backlit bymeans of the illumination device according to the invention.

FIG. 2 shows a plan view through an exemplary embodiment,

FIG. 3 shows an enlarged illustration of one of the luminous spots,

FIG. 4 shows a sectional illustration of a luminous spot and of theparts of the exemplary embodiment which surround the luminous spot, and

FIG. 5 shows a schematic illustration of the power supply of the lightemitting diodes.

FIG. 1 schematically illustrates an arrangement having a light source 1and a display 3, an optical device for focusing the light emerging fromthe light source 1 with the aim of uniform distribution over the area ofthe display 3 being provided between the light source 1 and the display3. The distance between the light source and the display is a fewcentimeters. A heat sink 4 for heat dissipation is situated on the rearside of the light source 1.

FIG. 2 shows a plan view of the light source 1 having a white plasticframe containing 8×4 holes in the case of the exemplary embodimentillustrated, luminous spots 6 being situated in said holes. The size ofthe plastic frame 5 corresponds to the visible area of the display. Theplastic frame 5 and the luminous spots 6 are situated on an aluminumplate 7 that serves for fixing and heat dissipation. Lines in the formof a ribbon cable 8 for connecting the light emitting diodes to acurrent source are led out laterally.

FIG. 3 shows an enlarged illustration of a luminous spot 6 formed by ahole in the plastic frame 5. Four light emitting diodes 9 are arrangedin elevated fashion in the center of the hole on a submount 10 (FIG. 4).The light emitting diodes are connected via bonding wires 11 to lines14, which are only illustrated schematically by hatching of the areathat they occupy. In a preferred embodiment, one of the light emittingdiodes emits red light, two emit green light and the fourth emits bluelight. The light is mixed to form white in the case of this arrangement.The space between the submount 10 and the frame 5 is filled with a whitepotting composition 12, the surface 15 of which serves as a reflectorfor the light emitted laterally from the light emitting diodes 9. Atransparent potting composition 13 provides for a smooth surface of thelight source and protection of the bonding wires and light emittingdiodes.

FIG. 5 schematically illustrates the connection of the light emittingdiodes of an exemplary embodiment with 32 luminous spots to electriccircuits.

Since the multiplicity of connections can only be illustratedinadequately and confusingly in a drawing, the light emitting diodeswhose power supply is illustrated or described specifically areidentified by different symbols in accordance with their associationwith individual electric circuits. Two electric circuits forred-luminous light emitting diodes—also called red light emitting diodeshereinafter—are illustrated in detail. The columns of the grid arenumbered 1 to 8, while the rows are identified by the letters A to D. Todistinguish them from the reference symbols used elsewhere, the columnnumbers in FIG. 5 are printed in italics.

A respective one of the red light emitting diodes R of the luminousspots A1, A3, A5, A7, B2, B4 to D8 is connected to an electric circuit21, which furthermore contains a controllable current source 22 and acurrent measuring resistor 23. In the same way, the respective other redlight emitting diodes of the same luminous spots, namely A1, A3 to D8are connected to a further electric circuit 24 having a controllablecurrent source 25 and a current measuring resistor 26. The green lightemitting diodes of the luminous spots A1, A3, B2, B4, C1, C3, D2 and D4are correspondingly connected to two further electric circuits (notillustrated). Two further electric circuits (not illustrated) supply thegreen light emitting diodes of the luminous spots A5, A7, B6, B8, C5,C7, D6 and D8. The power supply of the luminous spots A2, A4, A6, A8 toD7 is effected correspondingly.

In the normal operating state, all the light emitting diodes are thussupplied with currents which are predetermined in such a way that theresulting light has the desired color. If one of the red light emittingdiodes connected to the electric circuit 21 then fails, by way ofexample, this is ascertained with the aid of the absent voltage drop atthe current measuring resistor 23 and the current source 25 iscontrolled with the aim of increasing the current in the electriccircuit 24. If this is not possible for reasons of the loading capacityof the affected light emitting diodes or the thermal balance of theindividual luminous spots, then it is possible to perform a reduction ofthe currents in the green light emitting diodes—which is not illustratedin FIG. 5.

The interleaving of the luminous spots means that structures which arisein the event of a change in the color and/or brightness of luminousspots connected to identical electric circuits become less visible andcan be distributed better by optical means than for example in the caseof a row- or column-type assignment of the luminous spots to therespective electric circuits.

1.-21. (canceled)
 22. An illumination device for backlighting an imagereproduction device containing light valves, the illumination devicecomprising a flat thermally conductive carrier and a plurality ofluminous spots arranged in a grid format on said carrier, each of saidluminous spots having a plurality of light emitting diodes and asubmount, each of said plurality of light emitting diodes of arespective one of said luminous spots being electrically insulated fromthe others of the light emitting diodes of the respective one of saidluminous spots, said submounts exhibiting good thermal conductivity andconnected to said carrier such that the connections between saidsubmounts and said carrier exhibit good thermal conductivity.
 23. Theillumination device of claim 1, wherein the area of said submounts isless than the entire area covered by the grid on said carrier, saidillumination device further comprising lines for supplying power to thelight emitting diodes arranged between said submounts on an insulatingcarrier on that area of the carrier not occupied by submounts.
 24. Theillumination device of claim 23, wherein said lines for supplying powerare routed in a flexible film that is continued as a flat lead outsidesaid carrier.
 25. The illumination device of claim 22, wherein saidsubmounts are made of silicon.
 26. The illumination device of claim 22,wherein said carrier is made of aluminum.
 27. The illumination device ofclaim 22, wherein said carrier is made of copper.
 28. The illuminationdevice of claim 22, further comprising a heat sink connected to saidcarrier.
 29. The illumination device of claim 23, wherein saidinsulating carrier comprises spaces between said submounts filled withplastic.
 30. The illumination device of claim 22, wherein said pluralityof light emitting diodes of a respective luminous spot emit varicoloredlight.
 31. The illumination device of claim 22, wherein each of saidplurality of luminous spots has four light emitting diodes.
 32. Theillumination device of claim 31, wherein said each of said plurality ofluminous spots has two green-luminous light emitting diodes, oneblue-luminous light emitting diode and one red-luminous light emittingdiode.
 33. The illumination device of claim 22, further comprising aplurality of reflectors, wherein a reflector surrounds each respectiveone of said luminous spots.
 34. The illumination device of claim 33,wherein each reflector forms a depression that is filled with atransparent plastic.
 35. The illumination device of claim 22, wherein arespective one of said plurality of light emitting diodes of oneluminous spot is connected in series with a respective light emittingdiode of another one of said plurality of luminous spots and forms anelectric circuit.
 36. The illumination device of claim 35, wherein theones of said luminous spots having said light emitting diodes associatedwith said electric circuit are interleaved with luminous spotsassociated with at least one other electric circuit.
 37. Theillumination device of claims 35, wherein each luminous spot includes aplurality of identically colored light emitting diodes, each of theidentically colored light emitting diodes being connected to differentelectric circuits.
 38. The illumination device of claim 37, furthercomprising control devices arranged and dimensioned for providingcurrents fed to each of the electric circuits, wherein said controldevices, in the event of interruption of one of the electric circuitsfor said identically colored light emitting diodes which causes a colorshift in the color produced by said luminous spot, control the currentsin the electric circuits for the at least one other electric circuit forthe identically colored light emitting diodes or for differently coloredlight emitting diodes of the same luminous spots to compensate for thecolor shift produced by the interruption.
 39. The illumination device ofclaim 38, wherein the control of the current comprises an increase inthe current in the at least one other electric circuit for identicallycolored light emitting diodes.
 40. The illumination device of claim 38,wherein the control of the current comprises a decrease in the currentin the at least one other electric circuit for differently colored lightemitting diodes.
 41. The illumination device of claim 35, wherein saidplurality of luminous spots form a grid of 4×8 luminous spots, each ofsaid plurality of luminous spots having two green-luminous lightemitting diodes and two red-luminous light emitting diodes, wherein fourelectric circuits are provided for the red-luminous light emittingdiodes, two of said four electric circuits being assigned to saidred-luminous light emitting diodes of identical luminous spots, saididentical spots being distributed over the grid in checkered fashion.42. The illumination device of claim 41, wherein each of saidgreen-luminous light emitting diodes are connected to eight electriccircuits, in each case one green-luminous light emitting diode of eightluminous spots being connected to one electric circuit and a furthergreen-luminous light emitting diode of the same luminous spot beingconnected to another electric circuit.
 43. The illumination device ofclaim 22, wherein said carrier is composed of ultrapure aluminum. 44.The illumination device of claims 35, wherein each luminous spotincludes a plurality of identically colored light emitting diodes, eachof the identically colored light emitting diodes being connected todifferent electric circuits.